The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Dye Sensitized Solar Cells (DSSCs), developed by O'Regan and Gratzel in 1991, are third-generation photovolatic devices which offer increased photoelectric transformation efficiencies at a low monetary cost. In contrast to silicon-based photovoltaic systems, where a semiconductor assumes both the tasks of light absorption and charge carrier transport, the configuration of a dye sensitized solar cell separates these functions.
Dye Sensitized Solar Cells (DSSCs) are unique in their functions in that they mimic the naturally occurring photosynthetic process found in plants and green algae. DSSCs utilize chromophores, similar to a photosynthetic system's porphyrins, in order to generate photosensitive charges which convert sunlight to energy. When subjected to light of an appropriate wavelength, these chromophores, as part of a dye molecule, are photochemically excited, and serve as the basis for conversion of sunlight into electrical energy.
Structurally, a dye sensitized solar cell (DSSC) has four main components which include 1) an optically transparent electrode (anode); 2) an organic, or organometallic, molecule (known as a dye or photosensitizer) adsorbed on a semiconductor oxide; 3) a liquid inorganic electrolyte or a solid organic hole-transporting material; and 4) a counter-electrode (cathode). The anode and cathode are arranged in a sandwich-like configuration, and the electrolyte is inserted between the two electrodes.
For a DSSC to operate the dye must allow electrons to move to an orbital with a higher energy (a dye excited state). The movement of these electrons result in the occurrence of a charge separation at the interface of the dye (now sensitized) and the semiconductor oxide components. This charge separation occurs via a photo-induced electron injection from the dye into the conduction band of the semiconductor oxide (i.e. titanium dioxide), leaving the dye molecules in an oxidized form. The electrons are then collected on a transparent conductive layer, and reach the counter-electrode (cathode) through an external electric circuit. The oxidized molecules of the dye are then regenerated through a transfer, such as one catalyzed by platinum (Pt), deposited on a cathode. Herein, the electrons trigger a series of redox reactions mediated through a redox pair which acts as an electrolyte. At the end of the reactions, the redox pair, now in a reduced form, transfers an electron to the dye, (which was held in an oxidized form), subsequently regenerating it and closing the cycle.
In addition to their reduced production costs and potential for achieving high energy-conversion efficiencies (η). Dye sensitized solar cells (DSSCs) have also attracted considerable attention for their flexibility, semi-transparancy, and stability during both prolonged light and thermal stresses. DSSCs have a greater independence on the angle of incident light needed to operate, as well as a high response to low level lighting conditions. As such, DSSCs tend to outperform conventional silicon photovoltaic devices under diffuse lighting conditions, such as those found in shady areas, on cloudy and/or rainy days, or even indoors with ambient lighting conditions. Indoor applications for DSSCs include their incorporation into a variety of devices such as, but not limited to, cell phones, laptop computers, and ipads. DSSCs may also be utilized in conventional solar arrays, or for building-integrated photovoltaic products such as windows, skylights, solar tubes, and siding.
For solar cells to attain the desired high level of photoelectric transformation efficiency (η), the dye component should possess a wide absorption spectrum (of solar light), as well as a high molar extinction coefficient (ε). Consequently, photoelectric transformation efficiency is primarily determined by the number of collected and injected photons, and thus by the light absorbed by the dye or photosensitizer. Additionally, for an efficient electron injection, the dye must be able to absorb (by chemisorption) onto the surface of the semiconductor, and upon photoexcitation, inject electrons into the conduction band of the semiconductor with a quantum yield of unity. As known in the art, the ‘quantum yield of unity’ is useful in modeling photosynthesis, where QD=QA=1.
Currently, the most successful and widely-used dyes are organometallic compounds based on ruthenium 2+ complexes [Ru(II)]. These dyes have allowed photoelectric transformation efficiencies (η) of 11% to be reached. However, despite their efficiency, ruthenium compounds have a variety of disadvantages. One concern is the low molar extinction coefficient (ε) inherent to derivatives of ruthenium [Ru(II)]. Other shortcomings involve the expensive and complex synthesis and purification phases which ultimately result in a product having a limited chemical stability.
Further still, the photo-active regions of photovoltaic devices employing ruthenium complex dyes are reduced to the visible part of the solar spectrum, and within that, to the shorter wavelength regions. Consequently, photons of the longer wavelength regions are not harvested, and cannot be converted into electrical energy. Therefore, it is desirable to extend the photo-response of a dye into the longer wavelength regions of the solar spectrum so as to improve the overall light-to-electricity conversion efficiency of a DSSC. As synthetic modifications of ruthenium complexes have been carried out with limited success, the development of alternative dyes with wider absorption bands falling within the longer wavelength regions of the solar spectrum has garnered a greater interest.
Metal-free organic dyes present one such alternative dye. As a group, they present advantages over organometallic dyes for use in DSSCs including, but not limited to: (1) possessing high molar extinction coefficients (ε), (2) having simpler and less expensive synthesis processes, and (3) the existence of a chemical industry already capable of carrying out a large scale synthesis process.
The structural arrangement of metal-free organic dye sensitizers is most often of the linear D-π-A type. Therein, ‘D’ is an electron donor group (i.e. electron-rich), ‘π’ is an unsaturated spacer having π-conjugated bonds and ‘A’ is an electron acceptor group (i.e. electron-poor group). Customarily, the electron acceptor group is further coupled with a group which allows for the anchoring (i.e. adsorption) of the dye molecule onto a surface such as titanium dioxide. The D-π-A system allows for a variety of modifications to be made to the dye by varying the D, π-spacer, and/or A groups.
For most metal-free organic dyes, arylamine derivatives function as the electron-donor group (D). These typically include groups of the triarylamine type (NAr3). A cyanoacrylic acid or rhodamine residue typically functions as an electron acceptor group A, while the π spacer is often based on thiophene structures, such as a fused monocyclic and/or polycyclic thiophene ring(s). With regard to the anchoring group, many organic dyes reporting good efficiencies have an anchoring group comprising an acrylic acid group. Often, a combination of the electron acceptor group ‘A’ and the anchor group include a 2-cyanoacrylate group. However, in the design of functional and efficient dye molecules, other groups should be considered for the D, π, and A groups.
The electrical and optical properties of π-conjugated organic molecules are linked to a parameter known as the HOMO-LUMO gap (Eg). This corresponds to the energy difference between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO). The energy difference between the HOMO and LUMO levels is approximately equal to the ‘band gap energy’ as defined for collections of molecules in a thin-film. This difference in value, given relative to the Fermi level and in units of Hartree, may serve as a measure of the excitability of a molecule. For example, the smaller the energy, the more easily the molecule will be excited. In order to be used in optoelectronics, including DSSCs, or photodynamic therapies, this band gap energy should be as low as possible.
It is also important to realize that many molecules presenting a low Eg gap are often those in possession of higher dimensions. These ‘high dimension molecules’ tend to, by virtue of their bulky structure, be weakly soluble, thus limiting their role in photovoltaic devices. Additionally, larger dimension molecules have been associated with greater toxicity, or health, risks to those individuals exposed to them. Therefore, it is important to find new light absorbing π-conjugated molecules (D-π-A) that present a low Eg gap, are soluble in a preferred solvent (i.e. an organic medium and/or water), and are non-toxic to humans and animals.
In summary, an important strategy for realizing a high photoelectric conversion efficiency is to allow for light of a long wavelength to be absorbed by the dye molecule, along with chromophore and a spacer group combinations which render a low HOMO-LUMO gap in the dye molecule as well.
In order to evaluate any proposed dye candidates, their properties, such as ‘band-gap energy’ should be determined. Dye energies, based upon the HOMO and LUMO value, can be established experimentally by means of cyclic voltammetry (CV), X-ray photoelectron spectroscopy (XPS) or ultraviolet photoelectron spectroscopy (UPS). Dye energies can also be calculated by means of quantum-mechanical methods, for example, by means of dependent density functional theory and/or time-dependent density functional theory (DFT/TD-DFT). Theoretical DFT/TD-DFT can suggest which dyes will have the greatest efficiencies prior to exploration of synthesis routes. Therefore, investigation into alternative D, π-spacer, and A groups is proceeding, with both theoretical and physical approaches, in order to determine dye energies and light harvesting efficiencies of candidate dye molecules.
In this regard, density functional theory/time dependent density functional theory (DFT/TD-DFT) is an effective tool for investigating the ground and excited state properties of photosensitizer complexes as compared to other high level quantum approaches. The DFT approach is, in principle, exact, with DFT/TD-DFT computed orbitals deemed suitable for a typical molecular orbital theoretical analysis and interpretation. Furthermore, DFT/TD-DFT fundamental scaling properties do not deteriorate when the methodological precision is increased.
Most dyes absorb light in the same range as a commonly used red-dye (below 600 nm). Thus photons of the longer wavelength region(s) are still lost to photoconversion. As the spectrum of solar radiation at ground level has an emission peak that extends from approximately 500 nm to approximately 650 nm, the use of dyes that have absorption peaks in this region of the spectrum are greatly needed. Dyes prepared to date either absorb below 500 nm, or above 650 nm, and therefore do not capture most of the solar radiation. A second drawback is that these dyes exhibit low molar extinction coefficients.
Other roles exist for light absorbing molecules, such as those photosensitizers which have been developed for DSSCs. These molecules may also play a role in treating medical conditions through a course of treatment known as phototherapy. Photodynamic therapy involves a minimally invasive two-step process wherein a photosensitizer is administered and, once it has permeated a target tissue, the photosensitizer is then activated by exposure to a dose of electromagnetic (usually light) radiation at a particular wavelength. Photodynamic therapies have been used to treat a number of conditions including the treatment of malignant skin cells, (ie basal cell carcinoma), age-related macular degeneration, actinic keratosis and hair loss.
More specifically, photodynamic therapy (PDT) is a form of phototherapy which uses nontoxic light-sensitive compounds that are exposed selectively to light, whereupon they cause a reduction in targeted hyperproliferative, malignant, and/or diseased cells. PTD may also result in the stimulus of hair growth. Key requirements for the design of effective phototherapeutic agents include, but are not limited to, having an efficient energy or electron transfer to cellular components, a low tendency to aggregate when placed in a solvent delivery system, an efficient and selective targeting of a desired tissue, a low systemic toxicity, and a lack of mutagenicity.
One phototherapy mechanism pathway occurs via the direct energy or electron transfer from a photosensitizer to a targeted cellular component(s) thereby causing cellular death or necrosis. The wavelength of the light source is required to be of an appropriate wavelength so as to excite the photosensitizer dye in order to destroy any tissues which have selectively taken up the photosensitizer and have been locally exposed to light. Malignant and other diseased cells of the skin (i.e. the epidermal, dermal and hypodermal layers) are the most accessible to light and can therefore be treated using photodynamic therapy with the greatest ease. As red light has a penetration of about 1 cm in living tissues, malignant and/or diseased cells on or near the surface of the skin can most preferably be treated with this wavelength of light.
Wavelengths of light in the near infrared region (NIR region 750 nm-950 nm) provide radiation which penetrates more deeply in the skin. Therefore, in order to treat malignant or diseased tissues which require a higher penetration of light into the body, the photosensitizer should absorb in these higher wavelengths. Also, as with the use of photosensitizers for DSSCs, the HOMO-LUMO (Eg) gap of the dye molecule(s) for use in a photodynamic therapy process should also be as low as possible.
Therefore, there is a need to provide photosensitizers which are versatile, have a low HOMO-LUMO (Eg) gap, and absorb light in a wide spectrum of wavelengths. Those compounds having absorption in the visible, UV and/or NIR region, i.e. between 300 nm and 1000 nm are desired. The development of dye compounds which absorb radiations from 400 nm-950 mn is highly desirable due to the fact that this specific range of absorbance enables the use of the dye compounds in the field of electronics, photovoltaics, optoelectronics and photodynamic therapies. An organic dye should also be resistant to photogdegradation, and contain a reactive group capable of binding the dye stably to a surface such as that of a semiconductor, or in the case of phototherapy, a layer of the skin, so as to facilitate the transfer of electrons.